Solo Flute is listed only by Irwin, who says: A name used to indicate that a certain 8' or 4' manual Flute stop is of prominent and probably loud voicing and suitable for following the solo line of notes. In the theater organs this may be a bigger-scale Tibia Clausa or Doppelflöte. The term is not used on other than theater organs. Wedgwood mentions it in passing, but has nothing to add other than the synonym Soloflöte.
Of the four examples of Solo Flute and six examples of Soloflöte listed in Osiris, all but two are in church organs, and none are in theatre organs. All are at 8' pitch except for one at 4' pitch.